HEAVY OILS (LUBRICATING OILS)
thermometer. The crucible is heated on a sand-bath so that the tempera-
ture rises about 5° per minute, a small flame being brought near to the top
of the crucible at regular intervals until a slight explosion occurs.
The results obtained by these two different methods vary in oils of normal
composition by 5-40° ; the method employed for the determination should
always be indicated.
6. Temperature of Ignition.—This determination may be made as
the complement of that of the flash point—when an open crucible is employed
—by continuing to heat so that the temperature rises from 2° to 6° per
minute ; a flame is held for I -2 seconds near the surface of the liquid, the
temperature of ignition being taken as that at which the surface of the oil
The temperature of ignition is usually 20-60° above the flash point.
7. Viscosity.—This is determined by means of viscometers, that of
Engler (Fig. 48) being the one most used. It consists of a covered cylindrical
brass vessel A, the slightly sloping base of which is provided with a small
central aperture a leading to an efflux tube which can be shut by means
of a wooden plug b. In the vessel A are three points marking the level of
the liquid. The vessel A is surrounded by a larger one, the annular spaceously